Altidore broke the deadlock and Vazquez netted deep in injury time to settle Saturday's showpiece at BMO Field, as Toronto gained revenge for last season's penalty-shootout defeat in a mirror-image fixture, when Frei was the match-saving hero for Seattle.

Frei once again was the game's central figure, denying TFC time and time again, but his goal was breached for the first time this post-season 23 minutes from the end by Altidore before Vazquez converted from close range to make it 2-0.

Victory for Toronto adds the MLS Cup to the Supporters' Shield and Canadian Championship - making them the first side to win a clean sweep of domestic titles in MLS history.

After Greg Vanney's men set new a record for single-season points in 2017, their focus will now turn on becoming the dominant force in MLS, with Altidore backed up by difference-making displays from fellow star names Michael Bradley and Sebastian Giovinco.

Toronto made an electric start but once again found Frei nigh-on impossible to beat after the Swiss stopper's heroic display 12 months previously.

The goalkeeper plunged to his right to keep out Jonathan Osorio before closing the angles on Giovinco, who latched onto Vazquez's searching cross-field ball, but could not get enough purchase to lift the ball over Frei.

Seattle won last year despite failing to register a shot on target in 120 minutes and Frei kept them level to the break, tipping another further effort from Giovinco over the crossbar, although Joevin Jones did force Alexander Bono into action on the Sounders' only invasion of note into Toronto territory.

But Toronto ended the half well on top again, Frei denying Marco Delgado and Vazquez from distance before Jones almost turned into his own net while appealing for offside against Justin Morrow as Toronto's struggle to puncture Seattle's rigid defence began to frustrate.

Those frustrations came to the boil inside BMO Field 12 minutes after the break when Morrow went down under Kelvin Leerdam's ungainly challenge, but a penalty was not forthcoming from referee Allen Chapman.

The contest returned to a familiar feel as Frei dealt with Bradley's dipping strike and denied Giovinco's best effort yet, a scorching low shot destined for the bottom-left corner.

But the Italian's link-up play with Vazquez and Altidore came to the fore to finally break the deadlock.

Former Barcelona man Vazquez caught Seattle's defence high with a ball to Giovinco, whose quick reverse pass freed Altidore to power away from Roman Torres and lift a composed finish over Frei - finally beaten after 187 minutes of Cup action against TFC.

And the party atmosphere was ratcheted up in the fourth minute of stoppage time when Vazquez bundled home after Altidore struck the woodwork, Toronto securing a deserved win to cap a historic campaign.